Fluorescent lamps have "negative resistance." This means that the operating voltage decreases as power dissipation in the lamp increases. Therefore, circuits for supplying power to fluorescent lamps require a controllable alternating current power supply and a feedback loop to accurately monitor the current in the lamp in order to maintain operating stability of the circuit and to have an ability to vary the lamp brightness. Such circuits must be able to generate a high voltage to turn on the fluorescent lamp and then lower the voltage when current begins to flow in the lamp. If such a circuit is attempted to be operated when there is no lamp in the socket or a defective lamp is in the lamp socket, the voltage level in the socket will rise to a high level while waiting to sense a current in the absent or defective lamp. For purposes of this disclosure, a no lamp condition shall also mean a circuit having a lamp that is defective in a way to simulate an open circuit. This no lamp condition could damage circuit components and/or could be hazardous to humans. What is needed is a circuit for supplying power to a fluorescent lamp that senses when there is no lamp in the lamp socket and responds by shutting itself down.
Other devices have attempted to solve the problem of generating excessive voltage in the lamp socket. One such device is disclosed in Linear Technology's Application Note 55 by Jim Williams, entitled: "Techniques for 92% Efficient LCD Illumination." That device controls the output voltage when there is a open or broken lamp in the lamp socket by increasing the feedback in the lamp drive circuit. Another device is disclosed in a paper entitled: "Resonant Fluorescent Lamp Converter Provides Efficient and Compact Solution" by Mark Jordan and John A. O'Connor of Unitrode Integrated Circuits. That device disables the circuit outputs when an open lamp interrupts the current feedback loop. Both of these devices suffer from the disadvantage of continuing to consume excess power once an open lamp is detected. The Linear Technology device only limits the output voltage while allowing the lamp driver to continue operating. Further, because the Linear Technology device may put circuit components under higher thermal stress while limiting voltage at the transformer secondary, such components may need to be over-specified to survive this fault condition. The Unitrode device only disables the outputs while allowing the remainder of the circuit to continue operating. The Unitrode solution does not have means to limit the secondary transformer voltage during start-up, and could burn out a transformer during soft start at high supply voltages. The present invention does not consume excess power or suffer from these other disadvantages because it shuts down the entire lamp drive controller when a no lamp condition is present. This is an important feature to maximize the operating time of a device with a limited power supply capacity such as a battery powered lap top computer.